1. Field of the Invention
The present invention relates to an electron tube utilized as a photodetector for quantitatively measuring weak light. In particular, the present invention relates to an electron tube equipped with a sensing device having an electron entrance surface such as a semiconductor device for multiplying and outputting electrons emitted from a photocathode.
2. Related Background Art
There have conventionally been known electron tubes in which electrons emitted from a photocathode are accelerated and converged by an electron lens and then are made incident on a semiconductor device to yield a high gain. Such electron tubes are disclosed, for example, in U.S. Pat. No. 5,120,949, U.S. Pat. No. 5,374,826, and S. Base et al., "Test Results of the First Proximity Focused Hybrid Photodiode Detector Prototypes," Nuclear Instruments and Methods in Physics Research, A330 (1993), 93-99. In particular, the above-mentioned Base reference discloses an electron tube such as that shown in FIG. 1. This electron tube has an electrical insulating bulb 102 which secures electrical insulation between an anode 100 and a cathode electrode 101. The inner diameter of the cathode electrode 101 is made greater than that of the bulb 102, whereby a photocathode 103 has a large area, allowing a semiconductor device 104 to have an increased effective area (e.g., 100 mm.sup.2). Accordingly, it can be seen that the electron tube shown in FIG. 1 has a large size. The cathode electrode 101 employed in this electron tube is constituted by two pieces of cylindrical metal members 101a and 101b having inner diameters different from each other disposed concentrically with a gap therebetween.